To assist sea, air, and land navigation and other purposes, the United States Government has placed a number of satellites in orbit around the Earth in such a manner that, from any point on the Earth, a user operating a roving receiver may have a line of sight to at least four satellites. This system is referred to as the Global Positioning System (GPS). A GPS receiver receives GPS data from the satellites; from the GPS data the roving receiver can determine its position. The GPS data includes data regarding the position of the satellite. However, the GPS data is corrupted by the U.S. Government in order to degrade the accuracy of calculations performed. Such errors are easily eliminated using the proper decoding algorithms and codes; however, such information is only available to the U.S. Military. Also, atmospheric and meteorological conditions, electromagnetic interference from terrestrial sources and other satellites, kinematic motion or orientation of the roving receiver and other uncertainties further degrade the signals.
To ameliorate this problem, land-based reference stations at fixed, known locations have been erected to receive satellite transmissions and interpret the signals to generate measurement corrections, also referred to as DGPS (differential GPS) corrections. Using the true, known position of the receiver antenna at each reference station, these land-based reference stations derive measurement corrections that adjust the GPS data to produce more accurate results. These measurement corrections are transmitted, for example, via minimum shift keying (MSK) transmissions, to the roving receivers as deviations or offsets to be added to the measurements derived by the roving receiver from the GPS signals received directly from the satellites. An example of such a system is the Differential GPS NAVSTAR system operated by the U.S. Coast Guard to help ships navigate more accurately.
To ensure that the corrections being broadcast by DGPS stations are useful, i.e., that the corrections are not providing inaccurate position determinations at the roving receivers, integrity monitoring stations have been established. As shown in FIG. 1, a DGPS integrity monitoring (IM) station 10 includes a receiver unit 12 having both GPS receiver circuitry and integral radio receiver circuitry. In other instances, the GPS receiver circuitry and radio circuitry may each comprise separate units. In either case, the radio receiver portion of DGPS IM station 10 receives the DGPS corrections as they are being broadcast by the DGPS station 14 to rover units 22 and provides these corrections to the GPS receiver circuitry. The GPS receiver circuitry of DGPS IM station 10 also receives GPS signals from orbiting GPS satellites 16 in the conventional manner and computes its position using the GPS data from those signals and the DGPS corrections provided by the radio receiver circuitry. The position obtained as a result of these calculations is compared to a known reference position 18 of the IM station 10 (e.g., as determined from a precise survey) and an error which represents the difference between the GPS computed position of the IM station 10 from its known location is derived (e.g., by a computer system 20 associated with the IM station 10). If the error between the known location 18 and the GPS computed location of the IM station 10 station is not within acceptable user established tolerances, the IM station 10 may report an alarm condition to the DGPS station 14 operators. This may alert the operators of the DGPS station 14 that inaccurate DGPS corrections are being broadcast and that appropriate corrective action should be taken.
While DGPS techniques are suitable for applications requiring only sub-meter accuracy (e.g., shipboard navigation and the like), they are not suitable for applications requiring precise positioning (e.g., on the order of .+-.1 cm.) because of the techniques used to obtain this accuracy. Precise positioning applications, for example machine control applications and the like, require the use of real-time kinematic (RTK) GPS techniques. RTK receivers use locally collected GPS signals broadcast by GPS satellites along with reference carrier-phase and code-phase signals transmitted from RTK reference stations to compute position results down to the centimeter level. Unlike the DGPS corrections broadcast by DGPS reference stations, the signals transmitted by the RTK reference stations (hereafter referred to as RTK GPS data) are specially formatted messages which include various satellite observables (e.g., carrier phase and pseudorange measurements) as seen by the RTK reference station.
Rather than computing positions by simply developing pseudoranges to each visible satellite based on the times codes being transmitted by the satellites, extremely accurate GPS receivers, such as RTK GPS receivers, utilize phase measurements of the radio carriers received from various GPS satellites to compute positions. However, this position determination technique requires that so-called integer ambiguities be resolved by the GPS receiver. The integer ambiguities result from the fact that the receiver must compute the number of 360.degree. carrier phase shifts between itself and the GPS satellite, but each carrier cycle appears identical to the receiver. Sometimes, an RTK GPS receiver will produce a "bad fix" because the receiver failed to properly compute the correct number of integer phase shifts between itself and the GPS satellite(s). As a result, the receiver will report a ("bad") position that is based on a calculation which places the receiver either too close to or too far from the satellite.
As a result of the differences between RTK GPS receivers and other GPS receivers, the solution adopted by the DGPS community for integrity monitoring is unsuitable for RTK applications. To illustrate, consider that DGPS IM stations rely on the fact that the corrections being broadcast by a DGPS reference station are generally applicable to all roving GPS receivers operating in proximity to (e.g., up to approximately 300 miles from) the DGPS reference station. Thus, the DGPS IM station operating within a given area need only monitor the DGPS corrections being broadcast for that area and compute its GPS position accordingly. However, RTK GPS receivers must initialize to a selected group of integer carrier phase shifts for a selected group of satellites to obtain a position fix and there can be no guarantee that an RTK GPS receiver at an IM station has initialized to the same set of integer carrier phase shifts for these satellites as a roving receiver, especially if the roving receiver is operating in an area having a different visible sky from that seen at the RTK IM station. Because of these differences, position computations at a roving receiver may be different than position computations at an RTK GPS receiver at an IM station and, thus, the RTK IM station may not provide an accurate indication of the reliability of the RTK position at the rover. For these reasons, an improved integrity monitoring scheme for RTK GPS applications is desired.